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What Is Audio Compression? A Beginner’s Guide to Using Compression in Music
Compression is one of the most discussed concepts in music production – when to use it, what settings to choose, and, most importantly, what does it actually do? This article will answer just that, as well as explore the different types of compression and teach you a few tricks to try in your next mixing session. We’ve also prepared a free guide which you can download at the end of this article.
Put simply, compression controls the dynamics of a sound – how loud or quiet it is over time – helping to shape live recordings, balance different sounds, and add smoothness or punchiness to your track. Compression can be daunting – sometimes it can bring life to a mix, and sometimes it can do the opposite, squashing the natural sound into something unpleasant and robotic. This guide will break down exactly what compression does, what each control does, and when to use it, helping you to make more confident decisions for a more rounded mix.
The main goal of compression is to control the volume of a sound. More specifically, it gives you control over the dynamic range of a sound – the difference between the loudest and quietest sections. This makes it easier to balance multiple sounds and creates a consistency that helps you avoid one sound (or one section of the song) becoming too loud and dominating the mix. A classic use case is for balancing vocals, by bringing down the volume of the louder parts so that the whole performance sits more evenly with the other sounds.
So, how does it work? The compressor sets a threshold level of volume (for example, -18dBs), and any time the sound goes over that level, it brings the volume back down below the threshold. Effectively, it makes the loudest parts of a sound closer in volume to the quieter parts, decreasing the dynamic range. This balance is important for clarity in a mix – vocals suddenly jumping up in volume, or percussion that randomly hits too loudly, can cover up the other sounds and distract the listener. Compression, when used right, smooths out these inconsistencies, allowing each track to fit more cleanly into the mix.
Another reason to use compression is character – certain compressors add colour to the sound in a musical, pleasant way, creating a feeling of warmth, glue or punch. This is often linked to the type of compressor that you use, as some are particularly designed to give these colouring effects.
Understanding the controls of a Compressor
Understanding how to use compression is all about knowing the controls. Most compressors, including hardware units, have the same basic parameters – threshold, ratio, attack, and release. Once you understand these controls, you can start making decisions with confidence and shaping your sound with precision.
- Threshold: The level of volume (dBs) that, once reached, causes the compressor to start working. Once the signal exceeds the threshold, the compressor reduces its volume. If the signal dips below the threshold, the compression stops, leaving the sound untouched. Lower thresholds mean more of the signal is compressed.
- Ratio: The amount of compression applied to the signal once it reaches the threshold. For example, a 4:1 ratio means that for every 4dBs the signal goes above the threshold, only 1dB comes out. A higher ratio like 6:1 or 8:1 leads to more aggressive compression, which may make the sound noticeably reduced or ‘squashed’. In comparison, a lower ratio of 2:1 would only reduce the signal by 1dB for every 2dB it exceeds the threshold, which will tame the dynamic range in a much less obvious way, keeping most of the original signal’s range and ‘expressiveness’.
- Attack: How quickly the compressor kicks in once the threshold has been reached. A faster attack means the compressor acts more quickly to reduce the spikes, leading to a more balanced and ‘smooth’ sound. A slow attack does the opposite, letting the initial sound pass through before the compression begins. For example, an attack time of 0-10 milliseconds would reduce these peaks almost instantly, whereas 20-60 ms allows a lot more of the sound to pass through without the compressor affecting it, creating a volume spike or ‘punch’ at the start.
- Release: How quickly the compressor stops affecting the sound when it dips below the threshold. A fast release returns the signal to its normal level quickly, whilst a slower release keeps the compression going for longer. A fast release (50-100 ms) makes the effects of compression more noticeable, which can create a ‘pumping’ rhythmic effect. A slower release (200+ ms) makes the compression much smoother, so that the transition between compressed and uncompressed signal is less noticeable. This is useful for vocals or sustained instruments, as it keeps the signal more natural sounding.
- Make-up gain: This is just a gain knob, used for increasing the volume of the track after the compression. It’s usually used to match the volume of the compressed track to the original, pre-compression level, which is key for understanding how the sound has actually been affected – it’s easy to assume the compression made the sound worse, when in reality it’s just quieter. Level-matching means you are evaluating the sound, not the volume.
Types of Compressors
Modern, digital compressors are very good at affecting the dynamics without changing the character of the sound. This makes them useful for precise volume control without worrying about changing too much of the actual sound. Of course, digital (or in-the-box) mixing hasn’t always been an option, and today’s software compressors are largely based on yesterday’s hardware ones – big heavy boxes full of circuitry and analogue magic. These hardware compressors didn’t have the flexibility that modern software does, and were instead built for more specific uses. This gave them a unique flavor, color, and a distinct sound. Understanding how to use these different flavors of compressors is the next step in mastering compression and learning to choose the perfect tool to control and enhance your mix.
VCA Compression
VCA stands for Voltage Controlled Amplifier. These compressors are versatile, precise, and usually have a fast attack. They’re best for controlling peaks, whether that’s harsh percussive hits, uneven vocals, or just generally smoothing out a mix.
Pros: Clean, versatile, not too obtrusive
Common uses: Smoothing out key sounds, subtle compression for overall balance
Examples: SSL Bus compressor, API 2500
FET Compression
FET stands for Field Effect Transistor, and is the most aggressive, fast-acting type of compressor. They clamp down quickly when the volume spikes, which means the overall track volume can be raised without clipping or obscuring other sounds, making them key for adding energy and power to a mix.
Pros: Quick attack, strong control of dynamics, clean
Common uses: Vocals, snares, and loud percussive hits, parallel compression
Examples: UAD 1176, Waves CLA 76
Optical Compression
Optical compressors use a light sensor that is triggered when the threshold is exceeded. This makes them slow to react, which is equivalent to setting a slow attack time, giving a polished and gentle reaction to the changes in dynamics.
Pros: Smooth, natural sound, gentle dynamic control
Common uses: Vocals, bass, acoustic instruments
Examples: Teletronix LA-2A, Tube-Tech CL 2A
Vari-Mu Compression
Vari-Mu compressors add the most colour to a sound, as they typically use tube circuits (or tube-circuit emulations), which give the signal a vintage, analogue flavour. They typically have slower attack times than FET or VCA compressors, which makes them perfect for subtle control over multiple sounds at once, common in bus processing and mastering.
Pros: colourful, punchy, ‘glueing’ multiple sounds together
Common uses: adding vintage flavour, bus processing, and mastering
Examples: Manley Vari-Mu, Fairchild 670
Getting started with Compression
By now, you’ll have a solid grasp of the core controls and how to use them. That knowledge is definitely enough to start using compression effectively in your mixes – in fact, most of the results you’ll get from compression come from tweaking those basic controls with a good ear.
The best way to learn what compression sounds like is by testing the extremes of each control. Before you open your wallet and start looking at the higher-end compressors, grab a free plugin like TDR Kotelnikov, or open up your DAW’s native compressor. You’ll need an audio clip for testing purposes.
Start by turning the threshold all the way down, to around -50 dBs, and turning the ratio up high, around 10:1. You’ll hear the compressor clamping down hard – it’ll sound squashed and unnatural. Now adjust the attack from very fast (0-10 ms) to very slow (60+ ms), and notice how a slower attack creates a ‘punchy’ sound. Then experiment with the release-fast settings will lead to a ‘pumping’ effect, whilst slower ones will feel more natural. Now you’ve heard the extremes, try dialing in the settings to complement the audio, until the compression is more of a feeling than an obvious sound.
Let’s look at a real-world usage of compression – say you have a simple track with live drums, an acoustic guitar, and a main vocal. The drums are spikey, hitting too hard at certain points. The guitar is fairly consistent, but lacks impact, sitting too far in the background. The vocal is well recorded, but some words are sung too loudly, and the chorus is noticeably quieter than the verses. All of these sounds could benefit from compression, but what type of compressor (VCA, FET, Optical, or Vari-Mu) would be best for fixing these issues?
To work on a drum track with harsh spikes in volume, we’ll need a compressor with a fast attack and release, so that we can clamp down on the peaks without affecting the quieter sounds. A VCA compressor does this pretty well, but what we really need is a FET compressor to clamp down on the spikes quickly.
To give it more presence in the mix, the acoustic guitar needs a boost to the ‘punch’ to help it cut through. To do this, we need a slower attack to preserve the ‘punch’, making the Optical compressor perfect for retaining those acoustic plucks whilst smoothing out the sound. If we want to add some flavour to enhance the guitar tone, a Vari-Mu compressor could work too, especially if the goal is a softer, more vintage sound.
The vocals have two problems: some words are noticeably louder, and the overall dynamic range is too wide. The pro move here is to use two compressors. First, either a FET or VCA compressor would work due to their faster attack and release times. The second could be an Optical compressor or a Vari-Mu, as both have slower attack times and a smoother, more natural sound.
Advanced Controls of Compressors
Once you’re comfortable using the basics, you’ll start noticing some compressors have extra settings that can give you even more control over the sound. These advanced features are the next step in fine-tuning, solving unique problems and getting creative in your use of compression. There’s almost no limit to the extras that a plugin designer may include, but here are some of the key ones you’ll see more often.
- Knee: This controls how gradually compression kicks in. Unlike attack, which just affects when the compression begins, knee changes how smoothly it ramps up. A hard knee will apply the full compression instantly, whereas a soft knee introduces a curve where the sound becomes gradually more compressed. Most software compressors are hard knee by default, but the soft knee is often used for even more smoothness when compressing a sound.
- Auto-gain: Instead of manually setting the make-up gain, auto-gain automatically detects how much volume is reduced by the compression and adds it back on. This is useful for saving time and not having to constantly monitor the changes in volume. However, most auto-gain settings are imperfect and ignore how your sound fits into the context of the mix, so for precise control, manually setting make-up gain is usually better.
- Sidechain: This allows you to trigger compression on one sound using another. For example, a guitar track could have compression triggered by the vocal, meaning whenever the vocal exceeds the threshold, the guitar ducks out of the way. The ratio, attack and release still apply to the guitar track, but only become active when the secondary (or ‘sidechained) signal becomes louder than the threshold. This is great for creating space in a mix – this way the guitar only ducks down when the vocal reaches a certain volume, allowing it to play at full volume in sections without the vocal. The most common use of this is for ‘ducking’ the bass out of the way of the kick drum, so the kick isn’t hidden by the bass, and the bass keeps its power when the kick isn’t playing.
- Multi-band: Multi-band compression splits the sound into different frequency bands – typically the low frequencies, mids, and highs. This wa,y each band can be compressed individually. This is very useful for complex, multi-track recordings, such as an audio file of a whole drum kit being played – you can separate the low frequency kick, mid frequency snar,e and high frequency cymbals and control them individually.
- Lookahead: This is a high-end feature present in the most high-end surgical compressors. It enables the compressor to ‘look ahead’ at the audio that will be played next, so that it can catch any audio that exceeds the threshold as soon as it occurs. This results in a cleaner, more natural sound, but the effect is very subtle, making this most useful when the compressor is working on multiple sounds at once, like in a mix bus or in mastering.
- Mix knob/ Parallel compression: The mix knob is used to blend the original, uncompressed signal with the new, compressed signal, to find a balance between compression and natural dynamics. A popular trick is parallel compression (often referred to as New York compression), where a heavily compressed (low threshold, high ratio) signal is blended with the uncompressed signal, balancing the best of both worlds – bringing up the quieter details and adding ‘punch’ or ‘pumping’ with the compression while keeping the natural sound of the original.
Using Compression going forward
In conclusion, compression is one of the most powerful tools in the mixing engineer’s arsenal, and absolutely key for managing the balance and clarity of a mix. By learning how the core controls work, experimenting with how they sound, and exploring the more advanced features, you’ll gain the knowledge and ear-training needed to make confident and informed decisions in your next mixing session. As with all production tools, the key is to use your ears and trust your instincts – a Vari-Mu compressor may be recommended for bus processing, but if you find better results with a VCA or FET compressor, then that’s the tool for the job. The more you practice and experiment, the more intuitive it’ll become, and soon compression will be a go-to tool for all your mixes.
FREE Guide to Compression [DOWNLOAD PDF]
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Compression Cheatsheet
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